Regular sub-wavelength ripples formation by femtosecond laser pulses on
silicon
Yoshiharu Namba, Litao Qi
Motohiro Yasui, Kazuhiro Nishii, Hikoharu Aoki (Brother Industries, LTD.)
In this research, we study the regular sub-wavelength laser induced periodic
surface structures (LIPSS) formation on the silicon surface by femtosecond laser
pulses. A 780-nm-wavelength femtosecond laser, through a 0.2 mm pinhole aperture
for truncating fluence distribution, was focused onto the silicon surface. LIPSS
with a spatial period of 710 nm on the entire ablated area are obtained. The
initiation and evolution of LIPSS formation are observed by AFM and SEM to well
understand the mechanism of LIPSS formation. We find that the ablation mechanism
of mass removal before and after the LIPSS formation is changed. Phase explosion
play an important role on the mass removal during the LIPSS formation. Depth of
regular LIPSS on silicon is measured by AFM and is found to keep a saturated value
after regular LIPSS formation. The composition of LIPSS is discussed. Finally,
effect of the crystal orientation on the LIPSS formation is evaluated.
１．Introduction
Ultrafast lasers offer a number of advantages in materials processing such as high energy
flux and limited thermal effects leading to locally confined structural modification [1].
The semiconductor material silicon is of paramount important in microelectronics [2].
Femtosecond laser ablation of silicon is widely studied due to a number of phenomena concerning
photo-induced modification of silicon surfaces, such as ripple formation, column growth and
crater formation due to materials removal [3, 4]. Though these phenomena can limit the
precision of micromachining, some researches are carried out on the controlled manufactured
silicon ripples [5] and microcolumns [6] due to their potential applications such as
improvement on the absorptivity and field-emission source in the display technology [7, 8].
High quality micromachining processes by nonthermal ablation are attractive applications of
femtosecond lasers but are limited by their low speed manufacturing due to relatively small
available average power of the femtosecond laser system, typically several watts. The large
area treatments using these phenomena by femtosecond laser pulses are expected to be more
practical as the femtosecond fiber laser with high repetition rate is developed.
Ripples formation, also termed laser-induced periodic surface structures (LIPSS) were first
observed by Birnbaum on various semiconductor surfaces [9]. Since then, LIPSS on solid
materials have been studied extensively [5, 10-13]. The mechanism of LIPSS formation due to
the interference between the incident beam and the scatter waves is widely accepted. Recently,
self-organization is also proposed as a potential mechanism of LIPSS formation [5] due to
the LIPSS similar to the structures induced by ion sputtering [1] and dune formation in desert.
Though the physics of ripple formation are studied worldwide, the initiation and growth
mechanism still contain unanswered questions. The mechanism of mass removal during femtosecond
laser ablation varies with different materials and experimental conditions. Several
explanations have been proposed, such as non-thermal ablation [14], direct solid-vapor
transition [15], phase explosion [16-18], and Coulomb explosion [19, 20]. The initiation and
evolution of LIPSS formation are also affected by the mechanism of mass removal during
femtosecond laser ablation. Knowing the mechanism of mass removal during LIPSS formation will
be beneficial to understand the mechanism of LIPSS formation and expand their applications.
Veld and Veer have reported the initiation of ripples in steel by the femtosecond laser pulses
[21]. In this paper, we studied the formation of regular ripples formation on the silicon
surface by femtosecond laser pulses. On the ripple formation, we have demonstrated a method
to obtain the regular LIPSS on the entire ablated area [22]. In the experiment, the same method
is used and regular LIPSS are obtained on the entire ablated area on silicon by femtosecond
laser pulses. The process strongly depends on the laser fluence and the number of laser pulses
applied to the same spot. The ablation mechanism of mass removal is investigated through
observation on the initiation and evolution of LIPSS formation by AFM and SEM. We find that
phase explosion play an important role on the mass removal during the LIPSS formation. Depth
of regular LIPSS on silicon is measured by AFM and is found to keep a saturated value after
regular LIPSS formation. The composition of LIPSS is discussed. Finally, effect of the crystal
orientation on the LIPSS formation is evaluated.
2．Experiment
In the experiment, we used a commercially available amplified Ti:sapphire laser system
(CyberLaser, IFRIT) that generated 164 fs laser pulses with a maximum pulse energy (Ep) of
1 mJ at a 1 kHz repetition rate and with a central wavelength λ = 780 nm. The horizontally
polarized laser beam was focused onto a vertically standing sample at a normal incidence.
The samples were mounted on an XYZ-translation stage. The laser beam had a Gaussian profile
with a diameter of 6 mm. To obtain a fine periodic ordering of surface nanostructures, the
laser beam, through a 0.2 mm pinhole aperture positioned near the 10× objective lens with
a NA = 0.3, was focused onto the sample. The laser spot size on the sample was approximately
~10 μm. The maximum pulse energy after the 0.2 mm aperture was 1.038 μJ. The number of laser
pulses, N, applied to the sample was controlled by a frequency control system. We studied
the initiation and evolution of LIPSS following irradiation with different number of laser
pulses. All experiments were carried out in air at atmospheric pressure and room temperature
in a cleanroom with Class 100. The material was the silicon wafer [P type, (100) and (111)
crystal orientation]. The morphology of the induced periodic structure was examined by
scanning electron microscopy (SEM). The surface profile was measured by atomic force
microscopy (AFM).
3. Results and discussion
The silicon surface was first irradiated by different pulse energy steps. When the pulse
energy is lower than 0.113 μJ, there is no ablation on the irradiated area. The laser fluence
of LIPSS formation on silicon by femtosecond laser pulses is around 0.29~0.59 J/cm2 which
is close to the laser fluence to obtain the ripples on the silicon surface by Bonse et al.
[3]. As the laser fluence increases, microcolumn and the crater are formed on the irradiated
area by femtosecond laser pulses.
3.1 LIPSS formation on the entire ablated area
The LIPSS formed on the silicon surface by femtosecond laser pulses with a 0.2 mm aperture
following ablation at different number of laser pulses are shown in Fig.1. From the Fig.1,
the LIPSS on the entire ablated area are obtained. The orientation of LIPSS is found to be
perpendicular to the incident polarization. The mechanism of LIPSS formation due to the
interference between the incident beam and the scatter waves has been widely accepted. The
LIPSS formed on the entire ablated area result from the truncated fluence distributions of
the laser beam, cutoff in terms of the local fluence and lack of intensity fluctuations [22].
Fig.1. Regular LIPSS formation on the silicon surface irradiated at Ep = 0.176 μJ and different
number of laser pulses.
3.2 Initiation and evolution of LIPSS on silicon
The initiation and evolution of LIPSS on the silicon surface were evaluated by AFM and SEM.
Periodic surface structures appeared only after several consecutive pulses, which were found
to depend on the type of material, laser fluence, number of laser pulses, and wavelength [3,
13, 22, 23]. Figure 2 shows the evolution of the laser irradiated area by femtosecond laser
pulse at Ep = 0.158 μJ and different number of laser pulses. At N < 10, no ripples are formed
and several rings are observed on the laser irradiated area. The rings formation on the laser
irradiated area has been studied by Linde et al. and they found that the ablation during the
rings formation was characterized as a rapid thermal process and the characteristic states
of the conversion of solid material into hot fluid matter [24]. No particles are found around
the laser irradiated area during the rings formation. At 10 < N < 25, the laser irradiated
area is characterized by rings(see Fig.2(a) and 2(b)) or ripples (see Fig.2(c) and 2(d)) at
intervals under the same experimental conditions. The compared results of the laser irradiated
area under the same experimental condition are shown in Fig.3. Two distinct features are
existed between the rings and ripples formation. One is the significant change in the ablation
depth. The rings have the ablation depth of several nanometer or tens of nanometer (as shown
in Fig. 2(a), 2(b), and Fig. 3(a)) and the ripples have the ablation depth of several hundreds
of nanometer (as shown in Fig. 2(c), 2(d) and Fig. 3(b)). The other is that a large number
of particles are observed around the ripples whereas no particles are found around the rings.
The initiation and evolution of ripples on the silicon surface by femtosecond laser pulses
are different from that in metals presented by several research groups [21-23]. The initiation
and evolution of ripples in metals are the sequence of nanoprotrusions (bubbles), pre-ripples,
regular course ripples and regular ripples.
At N > 25, the ripples are formed on the entire ablated area and are shown in Fig.4. Then
as the laser shot increases, laser induced surface structures entirely fill the ablated area.
At N > 250 shots, LIPSS start to disappear gradually in the central spot area. At N = 1000,
LIPSS disappear and microcolumns and microholes are filled with the ablated area. During the
formation of LIPSS on the silicon surface by femtosecond laser pulses, large numbers of the
particle are observed around the LIPSS. The amount of the particle increases with the number
of laser pulses. The particles induced by femtosecond laser irradiation around the LIPSS on
the silicon surface are shown in Fig.5. The diameter of the particle is around 100 nm. As
the number of laser pulses increased, the particles become larger as shown in Fig. 4(f). Figure
6 shows the evolution of the LIPSS on the ablated areas at different pulse energies and the
same laser shots. As the pulse energy increases, periodic patterns, regular LIPSS, and
microcolumns are generated on the laser ablated area, sequentially, which are in agreement
with the results presented by Bonse et al.[3].
Fig.2. Evolution of the laser irradiated area by femtosecond laser pulses on the silicon
surface at Ep = 0.158 μJ and different number of laser pulses.
Fig.3. AFM photos of the laser irradiation area on the silicon surface by femtosecond laser
pulses at Ep = 0.158 μJ and N = 17.
3.3 The mechanism of mass removal during the regular LIPSS formation
Phase explosion have been proposed as one of the mechanisms of femtosecond laser ablation
of silicon [16, 18]. Cavalleri et al. [18] studied the melting and ablation of silicon with
time of flight mass spectroscopy and observed several physical processes of thermal melting,
nonthermal melting, ablation and plasma formation as the laser fluences increased. Kelly and
Miotello [16] studied the contribution of vaporization and boiling to thermal-spike sputtering
by laser pulses using a completely classical model of thermal transport in a continuum and
identified that phase explosion was the main mechanism of femtosecond laser ablation. In our
experiment, the significant change in the ablation depth and large numbers of the particle
observation around the LIPSS indicate that a significant mass removal occurs on the laser
ablated area on the silicon surface by femtosecond laser pulses during the LIPSS formation.
These results suggest that the initiation of a different mechanism or a shift in the primary
mechanism responsible for mass removal, which are similar to the results on phase explosion
occurring during high power nanosecond laser ablation of silicon [25, 26].
Fig.4. Evolution of LIPSS on the ablated areas at Ep = 0.176 μJ and different numbers of laser
pulses.
Fig.5. Particles induced by femtosecond laser irradiation around the LIPSS on the silicon
surface at Ep = 0.176 μJ and 50 laser shots.
Cavalleri et al. [18] also found the laser fluences for transition from the liquid state
to the gas phase (Fabl<F<2Fabl), expansion occurring at temperatures higher than the critical
temperature (F > 2Fabl), and plasma formation (F > 5Fabl), respectively. The ablation threshold
fluences of the Gaussian laser beam can be calculated by measuring the diameter of the ablated
areas versus the pulse energy and extrapolating to zero [29]. It is known that for a Gaussian
spatial beam profile with a 1/e2-beam radius, ω0, the maximum laser fluence, F0, increases
linearly with the laser pulse energy, Ep,
F0 
2E p
 02
.
(1)
The squared diameters, D2, of the laser ablated craters are correlated with F0 by [29]
F
D 2  2 02 ln 0
 Fth

 .

(2)
Fig.6. Evolution of LIPSS on the ablated areas at different pulse energy and 50 laser shots.
Therefore, it is possible to determine the Gaussian beam spot size by measuring the diameters
of the ablated area, D, versus the applied pulse energies, Ep. Due to the linearity between
energy and maximum laser fluence, a 1/e2-beam radius of Gaussian beam, ω0, can be determined
by a linear least squares fit in the representation of D2 as a function of ln(Ep). The Gaussian
beam spot size can be obtained. With this value, the maximum laser fluence 0 could be calculated
from the pulse energies in the experiment using Eq.(1). The threshold fluence Fth could be
obtained by an extrapolation to D2=0. The result for single pulse laser ablation is shown
in Fig.7. The ablation threshold can be obtained. From the Fig.7, we obtain a trendlines by
linear least squares fit in the representation of D2 as a function of ln(Ep). The beam spots
size and ablation threshold is obtained (see Fig.7). The ablation threshold fluence of
femtosecond laser ablation of silicon by single laser pulse irradiation is 0.26 J/cm2. The
ablation threshold decreases with the number of laser pulses increasing due to the incubation
effects [3]. Bonse et al. [3] gave a empirical equation between the ablation threshold for
N pulses (F(N)) and the ablation threshold for single pulse (F(1))
F ( N )  F (1)  N  1 .
(3)
where ξ is a materials-dependent coefficient and is equal to 0.84 during femtosecond laser
ablation of silicon obtained by Bonse et al.[3]. In our experiment, we obtain the LIPSS on
the entire ablated area at N >25. Using the Eq.(3), we can obtain that the ablation threshold
fluence of 25 laser shots (Fabl(25)) is equal to 0.16 J/cm2. The laser fluence of LIPSS formation
was around 0.29 ~ 0.59 J/cm2. So we can obtained the relationship between the ablation threshold
fluence (Fabl(N)) and the laser fluence of LIPSS formation (FLIPSS): 1.8Fabl(N)<FLIPSS<4Fabl(N).
As the number of laser pulses increases, the laser ablation threshold for N laser shots
decreases. It is easy to fulfill the condition of FLIPSS>2Fabl(N) which corresponds with the
results that expansion occurring at temperature higher than the critical temperature presented
by Cavalleri et al. [18]. From the analysis of the experimental results, we confer that phase
explosion is the main mechanism of femtosecond laser ablation of silicon during regular LIPSS
formation. It is to be stressed that homogeneous boiling (phase explosion) occurs over the
whole liquid layer and not only the surface as heterogeneous boiling [18, 27, 28]. It is easy
to understand the significant change in ablation depth before and after the ripple formation.
The significant change in ablation depth during the ripple formation is due to the phase
explosion occurring over the whole liquid layer, while the thinner ablation depth during the
ring formation is due to the normal boiling or vaporization only occurring over the surface
of the laser irradiated area.
Fig.7. Squared diameter D 2 of the ablated area versus the applied pulse energy Ep on femtosecond
laser ablation of silicon by single laser pulse irradiation.
3.4 Spatial period and depth of LIPSS formed on silicon
The profile of regular LIPSS on the silicon surface is shown in Fig.8. The typical feature
size of regular LIPSS is around 710 nm. The spatial period of LIPSS does not precisely
correspond to the laser wavelength and is a little smaller than the laser wavelength. The
deviation of the spatial period from the laser wavelength was attributed to the optic parameter
variation due to the surface plasmons excited by the femtosecond laser irradiation of the
surface [23]. The LIPSS formation on the silicon surface has the same level with the surface
of the sample, as shown in Fig.8; whereas, the LIPSS formation on the stainless steel surface
with a depth of about 250 nm was on the surface of the ablated crater with a depth of about
700 nm [22]. The compared results of LIPSS formation on the silicon and stainless steel surface
by femtosecond laser pulses is shown in Fig.9. The formation of LIPSS is attributed to the
interference between the incident light and a surface wave (generated by scattering). This
interference leads to periodic modulation of the absorbed intensity and consequently to
modulated ablation. But the LIPSS formation on different materials also has relevance of the
mechanism of femtosecond laser ablation of different materials.
Until now, more researches are focused on the spatial period of the LIPSS. LIPSS with a spatial
period much smaller than the laser wavelength on different materials by femtosecond laser
pulses have been reported [13, 22, 30-35]. However, few researches are carried out on finding
the relationship between the depth of LIPSS and the applied parameters. Normally, it is
difficult to obtain the depth of LIPSS which is sensitive to the pulse energy and the number
of laser pulses or only emerged at the edge the ablated craters. Regular LIPSS formed on the
entire ablated area is obtained in our experiment and then the depth of LIPSS on silicon is
measured by AFM. Regular LIPSS formation is very sensitive to the pulse energy, as shown in
Fig.6. Only the depth of the LIPSS on the silicon surface varying with the number of laser
pulses is presented, as shown in Fig.10. After 25 laser shots, the depths of regular LIPSS
on the silicon surface are around 362 ± 17 nm. Normally, the depths of the ablated crater
by femtosecond laser pulses increase with the number of laser pulses [36]. However, the depths
of regular LIPSS on silicon by femtosecond laser pulses keep a saturated value after N >25.
On the femtosecond laser ablation (or irradiation) of silicon, there are some special features.
For example, Izawa et al. found a ultrathin amorphous Si layer formed by femtosecond laser
pulse irradiation at low laser fluence and was quite uniform and did not depend on the laser
fluence and the number of irradiated laser pulses [37]; Rogers et al. found that the amorphous
Si layer inside the craters ablated by high power femtosecond laser pulses was absent and
a layer of a defective single crystalline Si was observed and was 400 nm thick on average
with a standard deviation of 200 nm [38]. Further experiments to clarify the relationship
between the depth of regular LIPSS and the subdamage lay will be carried out using TEM. More
investigations on the depth of LIPSS will be beneficial to well understand the mechanism of
LIPSS formation and expand the applications of LIPSS. The amorphous Si layer contribution
was weak and the polymorphs Si layer was dominated on the ripples zone [5]. In our experiment,
we find regular LIPSS after an ultrasonic cleaning become unclear and irregular. Further
experiments on the composition of LIPSS are being carried out and the results are useful to
understand the role of the self-organization mechanism in the LIPSS formation on silicon by
femtosecond laser pulses.
Fig.8. AFM photos of section profile of LIPSS on the silicon surface by femtosecond laser
pulses at Ep = 0.176 μJ and N = 25.
3.5 LIPSS formation on different crystal orientation of silicon
Experiment on the crystal orientation dependence of LIPSS formation on (100) and (111) Si
was carried out. The results show that the formation of LIPSS is independent of the crystal
orientation. The same result was reported by Borowiec and Haugen on the experiments on the
crystal orientation dependence of LIPSS formation on (100) and (110) GaAs [13]. From our
experiment, it is difficult to obtain the effect of the crystal orientation on the mechanism
of LIPSS formation. But the LIPSS formation independent of the crystal orientation could be
partially explained from the point in the mass removal. We carried out the experiments on
the effect of the crystal orientation on the diameter and ablation depth of the craters by
femtosecond single laser pulses on silicon. The results show that the crystal orientation
has little effects on the diameter and the depth of the ablated crater when the laser fluence
is below 5.42 J/cm2, as shown in Fig.11 and Fig.12.
Fig.9. AFM photos of 3D profiles of LIPSS formation on the silicon and stainless steel surfaces
by femtosecond laser pulses.
Fig.10. Relationship between the depth of regular LIPSS and the number of laser pulses on
the silicon surface by femtosecond laser pulses at Ep = 0.176 μJ.
Fig.11. Relationship between the diameter of the ablated craters and the laser fluence by
single femtosecond laser pulse irradiation on different crystal orientation of silicon.
Fig.12. Relationship between the ablation depth of the ablated craters and the laser fluence
by single femtosecond laser pulse irradiation on different crystal orientation of silicon.
4. Conclusions
Experiments on regular LIPSS formation on the silicon surface by femtosecond laser pulses
were carried out. The mechanism of LIPSS formation by femtosecond laser pulses was discussed.
The main conclusions are as follows:
1) Regular LIPSS with a spatial period of about 710 nm on the entire ablated area are obtained.
The LIPSS on the entire ablated area result from the truncated fluence distributions of
the laser beam, cutoff in terms of the local fluence and lack of intensity fluctuations
2) On initiation an evolution of LIPSS on silicon, the rings and ripples form in intervals
on the laser irradiated area when few numbers of laser pulses are used at low laser fluence.
Two distinct features are existed between the rings and ripples formation: a significant
change in the ablation depth and large number of the particle generation, which means the
initiation of a different mechanism or a shift in the primary mechanism responsible for
mass removal.
3) We confer that the ablation mechanism for mass removal is the phase explosion during LIPSS
formation on silicon by femtosecond laser pulses. The laser fluence for the LIPSS formation
(FLIPSS) has the following relationship with the laser ablation threshold fluence (Fabl):
1.8Fabl<FLIPSS<4Fabl, which meets the conditions for phase explosion occurring and avoids
plasma generation.
4) Depth of LIPSS on silicon by femtosecond laser pulses keeps a saturated value when regular
LIPSS are formed on the entire ablation area.
5) The formation of regular LIPSS on silicon by femtosecond laser pulses is independent of
the crystal orientation.
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